Why Are Whales So Big?

A blue whale, the largest known mammal ever known to exist on Earth, is seen swimming in Monterey Bay, California. Chase Dekker Wild-Life Images/Getty Images

If you've been ever been whale watching, visited a large aquarium or even seen the skeleton of a whale in a museum of natural history, you know the majestic massiveness of these aquatic mammals. The largest mammal to ever have lived on Earth in the history of the planet is not a dinosaur or some prehistoric monster. It's actually the blue whale, and it is alive right now, swimming around in the oceans.

Whales range in size from the massive blue whales, which can grow to more than 90 feet (27 meters) in length, to the relatively tiny pygmy sperm whales, which measure a measly 10 feet (3 meters) in length. But with all that ocean to swim around in, why aren't whales even bigger? It's not like they have to support their big bodies on legs and walk around. For that matter, though, why aren't they smaller?

So back to that question about why whales are big, but not too big. The study authors determined that there are two main factors why: heat loss and metabolism. Oceans can be pretty cold places to live, and whales — while very intelligent — don't really have the aquatic equivalent to thermal underwear.

So because they're warm-blooded mammals, they have to be large enough to keep from losing too much body heat to the surrounding water. Thermoregulation, then, keeps whales from being the size of, say, dogs. "When you're very small, you lose heat back into the water so fast, there's no way to eat enough food to keep up," study co-author Jonathan Payne, a professor of geological sciences at Stanford's School of Earth, Energy & Environmental Sciences, said in a press statement.

And speaking of eating, whales have to do it a lot. Like all mammals, they convert that food into energy for swimming, growing and doing other whale-like things. That's the metabolic system at work. But the researchers suggest that the metabolism of whales only gets faster as they get bigger, and so they can only get so large. "Basically, animals are machines that require energy to operate. This need for energy places hard limits on what animals can do and how big they can be," Craig McClain of the Louisiana Universities Marine Consortium, who co-authored the study, said in a press statement.

So it boils down to how much they can eat versus their metabolic rate that keeps whales from getting infinitely large. But what about those massive blue whales? They have baleen instead of teeth and strain their food instead of chewing it. So blue whales are not only the largest whales in the ocean, they're also the most efficient eaters of all.

Now That's Cool

Blue whales need to take in a lot of food and expend as little energy as possible in order to get so big. But they eat little shrimp-like creatures called krill that they strain through their baleen. Krill are only a few centimeters long, but they really add up — a blue whale eats about 4 tons (8,000 pounds) in a day.